The present invention relates to squarylium compounds, and filters for plasma display panels employing them. Particularly, it relates to filters for plasma display panels, which are characterized by having a layer containing a specific squarylium compound and which are capable of effectively screening neon emission radiated from plasma display panels.
In recent years, plasma display panels have been used as display panels of various electronic appliances including large size wall-hung TVs, and their demands have increased, and it is expected that their number will continuously increase in future.
In a plasma display, a gas mixture of xenon and neon is excited by electric discharge to radiate vacuum ultraviolet rays, and emission of three primary colors is obtained by utilizing emission of red, blue and green by the respective phosphors under excitation with the vacuum ultraviolet rays. At that time, so-called neon orange light having a center wavelength of about 600 nm will be emitted when neon atoms once excited, will return to the ground state (Journal of Institute of Image Information and Television Engineers, Vol. 51, NO. 4, P.459-463 (1997)). Therefore, the plasma display has a drawback that an orange color will be mixed to a red color, whereby a bright red color is hardly obtainable.
For example, JP-A-10-204304 discloses squarylium compounds which have only OH groups as substituents on the benzene rings. However, these compound have absorption at a wavelength departing from the wavelength region of the neon orange light, and they were inadequate to screen the neon orange light.
It is an object of the present invention to provide a filter for a plasma display panel, which is capable of effectively screening neon emission. Particularly, it is intended to provide a filter for a plasma display panel excellent in light resistance.
The present inventors have conducted various studies and have found that the above objects can be accomplished by using a compound containing a specific squarylium compound having hydroxyl groups.
Namely, the first gist of the present invention resides in a squarylium compound represented by the following formula (I), and a filter for a plasma display panel, characterized by having a layer containing such a compound, on a transparent substrate. 
[in the formula (I), R is a halogen atom, an alkyl group which may have a substituent, an alkoxy group which may have a substituent, or an alkenyl group which may have a substituent, m is an integer of from 1 to 4, and n is an integer of from 1 to 4.]
The squarylium compound represented by the above formula (I) is characterized in that it has OH groups together with other specific substituents on the benzene rings, whereby, as compared with a compound having only OH groups as substituents on the benzene rings, the absorption maximum is shifted towards the long wavelength side, and it has the absorption maximum in the vicinity of 600 nm (particularly from 580 to 600 nm) which is the wavelength of the neon emission, whereby the neon emission can effectively be screened, and it is possible to obtain an excellent filter for a plasma display panel.
Further, the present inventors have conducted various studies and have found it possible to substantially improve the light resistance of a filter for a plasma display panel by laminating an ultraviolet absorbing layer with a layer containing a specific squarylium compound having hydroxyl groups.
Namely, the second gist of the present invention resides in a filter for a plasma display panel, characterized in that a layer containing an ultraviolet absorber is further laminated on a layer containing a squarylium compound represented by the following formula 
[in the formula (Ixe2x80x2), R is a halogen atom, an alkyl group which may have a substituent, an alkoxy group which may have a substituent, or an alkenyl group which may have a substituent, mxe2x80x2 is an integer of from 1 to 4, and nxe2x80x2 is an integer of from 0 to 4.]
Further, the present inventors have found it possible to obtain a filter for a plasma display panel, having a better performance, by further providing a near infrared screening layer, an electromagnetic wave screening layer, an antireflection layer or a glare-preventing (non-glare) layer, in addition to the layer containing a squarylium compound, in the filter for a plasma display panel thus obtained.
Further, still another gist of the present invention relates to a plasma display panel device having such a filter for a plasma display panel on a screen of a plasma display panel.